The present invention relates to the protection of integrated circuits (ICs), and more particularly, to a scheme for preventing the reverse engineering of ICs. The invention is particularly useful in protecting secure ICs which are used in cable and satellite television decoders to prevent unauthorized users from receiving television broadcasts. The invention is equally useful in protecting secure ICs used in other applications, including terminals and smart cards for electronic funds transactions, premises access control, electronic games, and the like.
Due to the continuing popularity of the pay television market, there exists a tremendous financial motivation for unauthorized persons known as "pirates" to modify the access control of set-top boxes (e.g., decoders) to allow the reception of television programming without payment of the required subscription fees. The modified decoders are purchased by unscrupulous individuals through various markets and used to illegally receive and view the television signals.
To produce a modified decoder, a pirate must extract certain information from a genuine decoder, which is usually known only by the authorized manufacturer. The decoder typically includes a secure (e.g., cryptographic) processor which contains information such as cryptographic keys which are used in descrambling a scrambled television signal or other programming service signal (e.g., audio or data). Since the secure processor performs access control functions, it is a focal point of the pirate's attention. Accordingly, the pirate will employ various techniques in an attempt to gain information from the secure processor.
One common attack technique is known as "probing." A secure processor includes an integrated circuit (IC) which is fabricated as a monolithic device with an ensemble of active and passive components, including transistors, resistors, capacitors, and diodes which are interconnected within a monolithic block of semiconductor material. The IC may be located in a housing which is installed on a decoder board, or may be part of a micro-module which is embedded in a smart card. During probing, ICs such as very large scale integrated (VLSI) circuits are subject to an invasive attack wherein the die (e.g., IC or "chip") is exposed by decapsulation. During decapsulation, the compound material which encapsulates or surrounds the die is systematically removed. Then, probes which measure current and other parameters are used to monitor the electronic signals in the active components of the chip.
A pirate may perform the following decapsulation steps to prepare a chip for probing. First, the chip is removed from the decoder board, with the die still inside the IC package. Generally, this is the case when a chip is mounted on a large board. Second, the location of the die within the encapsulating compound of the IC package can be determined by taking an x-ray of the IC package. Third, a mechanical grinder can be used to remove as much of the encapsulating compound as possible above the top surface of the die without damaging the die. Fourth, chemical etching or plasma etching is performed to remove the last portions of encapsulating compound which remain over the area of the die which is to be probed. Some chemical etchants work so well on the encapsulating compound that the grinding step can often be skipped.
The decoder board may be a computer board such as those used in a personal computer (PC). Typically, the chip requires a direct current from a battery or other power source to maintain the contents of a volatile memory, such as a random access memory (RAM). In this case, the battery wires are soldered to a positive pin (e.g., V.sub.batt) and to a negative pin (e.g., V.sub.ss) on the outside of the chip prior to removal from the board. The chip is then removed from the board with the battery wires still attached. If the battery power is interrupted, the chip will "self-destruct" by erasing (e.g., losing) critical information stored in memory. The pirate can identify the appropriate battery pins by taking resistance measurements of the traces on the board which are connected to the battery, and then confirming by taking voltage readings of the traces with a main voltage (e.g., V.sub.cc) off.
The IC must be removed from the decoder board since the equipment used by the pirate in the subsequent decapsulizing steps cannot accommodate the large footprint of the large decoder board. Commercial decapsulation stations which use chemical etchants can accommodate only relatively small decoder boards. This is true since the IC must be placed in a chamber to protect the operator from the etchants, which can be toxic. Often, a small vacuum is created in the chamber to prevent etchant fluid and spray from escaping. Such chambers are typically limited in size due to the increased expense and time associated with evacuating a larger volume chamber.
Alternatively, the IC may be provided in a micro-module, which is plug-in, miniature circuit composed of micro-elements such as resistors, capacitors, and/or transistors. The die of the micro-module may be fabricated within a substrate matrix. One surface of the substrate matrix contains contacts, while the die is wire-bonded to the opposing surface. The micro-module is carried within a cavity of a card body. Commonly, a pirate extracts the micro-module from the card body and positions the micro-module to gain access to the contact area. The contact area is glued to a mounting surface, and the surface of the die is exposed to allow removal of the epoxy adhesive which is used to secure the die within the card body.
Thus, the pirate's task is made easier when the chip is consolidated into a smaller form factor of the chip itself or a much smaller board. The present invention is directed toward making it more difficult for the pirate to remove an IC from a board or micro-module, or to remove a micro-module from a smart card.
In current chip designs, including those for application specific ICs (ASICs), a pirate will generally not encounter significant obstacles in performing the four steps above. Removal of the chip from the board with the battery wires still attached is usually considered to be the most delicate operation. Thus, the undamaged die, which is protected by an inert glassivation coating, can be exposed by a pirate as long as a short circuit or open circuit of the battery power is not created. Furthermore, damage to bond wires can also easily be avoided. Bond wires connect bond pads of the chip to package pads in the protective encapsulating package, and are located on the periphery of the device. Once the die is exposed without causing either shorting or opening of the power supply to the chip, then probing can begin. If the removal of the chip from the board or micro-module that it is connected to can be prevented or otherwise hindered, then piracy may be made more difficult or avoided altogether.
One approach to deterring probing is discussed in commonly-assigned U.S. Pat. No. 4,933,898, issued Jun. 12, 1990 to Gilberg et al., entitled "Secure Integrated Circuit Chip With Conductive Shield." Gilberg et al. disclose using one or more conductive layers to overlay a secure area of an IC. The conductive layers shield the secure area from inspection and carry a power signal to the IC. Removal of one of the layers by a pirate causes the loss of power to the components of the secure area. However, this approach does not directly address the problem of removal of an IC from a decoder board or smart card.
Accordingly, it would be desirable to provide an apparatus which deters tampering of IC chips by a pirate. In particular, it would be desirable to make the pirate's task of removing an IC from a decoder board, micro-module substrate or smart card body without damaging the IC more difficult. Furthermore, the apparatus should be compatible with existing chip designs and inexpensive to implement.
It would also be desirable to provide an apparatus which makes focused ion beam deposition more difficult. Ion beam deposition devices are used to inject charged atoms or molecules into a die, and typically use a small evacuated chamber to hold the die. Evacuation of the chamber is made more difficult if the die is left on a board or micro-module substrate due to outgassing (e.g., the release of absorbed gases by heating) of the board or substrate. By making it more difficult and risky for a pirate to remove the die from the board or micro-module, the pirate's handling costs are increased since the required period to evacuate the chamber is increased.
The present invention provides an apparatus having the above and other advantages.